The last major eruption of Mount St. Helens, about 50 miles northeast of Portland, was in 1980. The mountain spewed steam and ash in 2004, and has since been rebuilding a new lava dome.
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New study co-authored by UW geologists looks at what lies below Mount St. Helens
The reason for the location of Mount St. Helens is an enigma. The volcano lies farther west than other peaks in the Cascades volcanic arc. Research published this week may begin to explain why.
The study was led by scientists at the University of New Mexico with co-authors at the University of Washington, Rice University and Cornell University. All are part of an ambitious effort to use remote sensing to better understand the hidden passageways beneath one of the country’s most dangerous active volcanoes.
The paper, published Nov. 1 in Nature Communications, analyzes compressional waves traveling through the crust and reflecting off the mantle below the volcano. Results show that on one side the mantle is largely serpentinite, a rare, moisture-absorbing, dark-green mineral that can look like a snake’s skin. But the mantle below the eastern half of the mountain is mostly olivine, a common mineral that allows water — thought to play a key role in volcanic eruptions — to percolate up and into the overlying crust.
The paper presents the latest results from a July 2014 experiment that conducted a giant ultrasound of Mount St. Helens. The National Science Foundation project, nicknamed iMUSH for imaging Magma Under St Helens, set off seismic waves to see how they travel under the mountain and generate a map of the volcano’s plumbing. The new paper is part of a larger collaboration that also involves researchers from the U.S. Geological Survey, Oregon State University and the Swiss Federal Institute of Technology in Zurich.
During that experiment, researchers from the University of New Mexico placed 900 autonomous seismographs within 15 kilometers (9.3 miles) of the crater, increasing the density of instruments right around the volcano. All sensors were deployed along the road and trail system at Mount St. Helens with an average spacing of 250 meters.
The iMUSH experiment set off 23 active-source explosions, with energy similar to small 2.0 magnitude earthquakes, over a two-week period. The resulting dataset provides high-resolution seismic imaging of deep crustal structure beneath this active arc volcano.
“We show that Mount St. Helens sits atop a sharp lateral boundary in Moho reflectivity,” said corresponding author Steven Hansen, a postdoctoral researcher at the University of New Mexico. The lack of reflections to the west can be explained if there is a relatively cold wedge of serpentinite to the west of the mountain, because the compressional wave speed of serpentine is not very different than that of the overriding crust.
In this context, cold is less than about 700 degrees C, or 1,300 degrees F. Below that temperature the serpentinite binds the water into a crystal structure. Above that temperature, however, serpentinite is not stable, and water can percolate up through the hot mantle unimpeded and into the overlying crust.
“The melt that supplies Mount St. Helens is probably formed to the east, in the mantle wedge below Mount Adams, and then moves west through the magmatic system somehow,” Hansen concluded.
UW co-authors are Ken Creager and John Vidale, both professors in the Department of Earth & Space Sciences. Other co-authors are Brandon Schmandt at the University of New Mexico, Alan Levander at Rice University, Eric Kiser at the University of Arizona and Geoff Abers at Cornell University.
“This is a nice result because it shows a very sharp boundary between where you have reflectivity and where you don’t, and that boundary between strong and weak reflectivity is pretty much directly beneath Mount St. Helens,” Creager said. “The density of data lets us see that this boundary between where there is reflectivity and where there isn’t is very sharp. Presumably what it’s telling us is the temperature of the mantle.”
Water is locked in various minerals inside the subducting oceanic plate. As the slab goes down, the temperature and pressure increase and the water is squeezed out of the crystals. The water then migrates up into the mantle of the overriding continental plate, where it reacts with olivine to become serpentinite to the west of Mount St. Helens, or olivine to the east. The temperature is key, Creager said, because that indicates where water in the descending ocean plate could be mixing with the rock to lower the melting temperature and form volcano-creating magma.
“An important question is: where is the water, and where isn’t it?” Creager said.
“This adds to a variety of other experiments that suggest that where it’s cold, this water is basically getting soaked into the mantle and turning olivine into serpentine and not going anywhere, so it can’t get up into the crust to form volcanoes,” he said. “When you get up into where there is olivine, the temperature is hotter, serpentine isn’t stable, so the water can play its role in the volcanic process.”
An iMUSH study published in the spring that was led by Rice University suggested that most of the eruptive products came from one or more chambers at depths of 3 to 12 kilometers (about 2 to 7 miles), and that those chambers may be connected.
The full iMUSH experiment includes four different lines of analysis. The UW-led team deployed 70 3-component seismometers that detected the tiny earthquakes that happen about twice a day, either due to movement of tectonic plates or motion of magma within the volcano, over a two-year period. UW researchers retrieved the instruments in August and are now analyzing their data. They hope to get a higher-resolution image of the deeper sections.
Overall, the goal is to create a more complete picture of the magma system beneath Mount St. Helens both to better understand and predict volcanic activity, and also to gauge the severity of the event when an eruption is imminent.
source : University of Washington – Seattle, WA